This invention relates to propylene block copolymer compositions, and more particularly to propylene block copolymer compositions which are suitable for a raw material for molded articles having excellent see-through property, transparency, stress-whitening resistance, stiffness and heat resistance.
Polypropylene resins have been used in a wide variety of the fields, because of their being relatively cheap and having excellent characteristics. In general, however, a propylene homopolymer has high stiffness, but poor impact resistance, especially impact resistance at low temperature. For improving the impact resistance at low temperature of propylene homopolymer, propylene block copolymer compositions have been provided wherein to propylene homopolymer component as initially produced is introduced an ethylene-propylene random copolymer component as subsequently produced. The propylene block copolymer compositions have been employed in various industrial fields including automobiles and household appliances.
The prior propylene block copolymer compositions have excellent impact resistance, but have the disadvantages that they are inferior in see-through property and transparency to homopolymer and an area stressed tends to whiten. As a method to alleviate the disadvantage of whitening by impact in the propylene block copolymer composition, there have been proposed a method of increasing the content of an ethylene unit in the copolymer component and a method of adding polyethylene to the propylene block copolymer composition. Both methods are excellent in an improvement of impact resistance, but reduce the transparency of the product.
JP-A-5-331327 discloses a polymer composition comprising a propylene block copolymer composition with the specified ratio of the intrinsic viscosity of a propylene homopolymer component to that of an ethylene propylene random copolymer component.
JP-A-6-145268 discloses a polymer composition with the specified intrinsic viscosity of a propylene homopolymer component, the specified ratio of the intrinsic viscosity of a propylene homopolymer component to that of an ethylene propylene copolymer component and the specified ethylene content in the ethylene propylene random copolymer component.
JP-A-56-72042 and JP-A-57-63350 disclose a polyolefin resin composition wherein an ethylene-propylene copolymer containing a small amount of ethylene is blended with other ethylene-propylene copolymer.
JP-A-10-87744 discloses a propylene type resin composition wherein a small amount of ethylene is incorporated into a propylene homopolymer component in the ethylene-propylene block copolymer.
These polymer compositions have been more improved than conventional propylene block copolymer compositions in respect of the stress-whitening resistance and transparency as seen from the result of stress-whitening test and the haze measured for the injection-molded articles of the polymer compositions, but more improvements have been required. Further, there is a problem of reducing see-through property of the articles, due to the presence of optical unevenness in the molded article which inhibits a direct advance of light. Therefore, more improvement has been desired.
In addition, blending various components may bring about unbalance of properties for each product, due to uneven dispersion of each component in the final product. This blending operation is not desirable.
An object of the invention is to provide a propylene block copolymer composition which is suitable for a raw material for molded articles having excellent see-through property, transparency, stress-whitening resistance, stiffness and heat resistance.
Other objects of the invention will be apparent from the description mentioned later.
As a result of various investigations to attain the above-described objects, we have found that a propylene block copolymer composition comprising (A) a propylene-xcex1-olefin random copolymer and (B) a propylene-xcex1-olefin random copolymer of different composition from the random copolymer (A) has excellent stress-whitening resistance and see-through property, when a composition of both random copolymers, an intrinsic viscosity of the propylene-xcex1-olefin random copolymer (B), the ratio of the intrinsic viscosities of both random copolymers and a product of the intrinsic viscosity ratio and the weight ratio of both random copolymers are within the prescribed ranges.
Thus the present invention relates to a propylene block copolymer composition comprising (A) a propylene-xcex1-olefin random copolymer with a propylene content of 99.4 to 99.9 mol % and (B) a propylene-xcex1-olefin random copolymer with a propylene content of 35 to 60 mol %, xcex1-olefin contained in the random copolymer (A) being ethylene and/or 1-butene, wherein the intrinsic viscosity ([xcex7B]) of the propylene-xcex1-olefin random copolymer (B) ranges from 0.5 to 2.0 dl/g, the ratio ([xcex7B]/[xcex7A]) of the intrinsic viscosity ([xcex7B]) of the propylene-xcex1-olefin random copolymer (B) to the intrinsic viscosity ([xcex7A]) of the propylene-xcex1-olefin random copolymer (A) ranges from 0.3 to 1.2 and the product (([xcex7B]/[xcex7A])xc3x97(WA/WB)) of the intrinsic viscosity ratio ([xcex7B]/[xcex7A]) and the weight ratio (WA/WB) of both copolymers ranges from 1.0 to 3.0.
The invention also relates to the propylene block copolymer composition wherein the propylene-xcex1-olefin random copolymer (B) contains not less than 80% by weight of a component soluble in xylene at 20xc2x0 C., based on the weight of the copolymer (B).
The invention further relates to the propylene block copolymer composition wherein the content of the propylene-xcex1-olefin random copolymer (B) is from 22 to 40% by weight, based on the weight of the propylene block copolymer composition.